The present invention generally relates to masks used for X-ray beam lithography and more particularly to a membrane used in the X-ray exposure mask or in a reticle for supporting a layer of X-ray absorbing material patterned according to a desired semiconductor device.
Generally, the membrane used in the X-ray exposure mask is required to have a high Young modulus, high tensile strength, high optical transparency, excellent resistance against radiation, and smooth surface. Currently, silicon carbide (SiC) is regarded as one of the most promising materials satisfying these requirements. Note that SiC has a Young modulus of 3-4 .times.10.sup.12 dyn/cm.sup.2 and a resistance against radiation of about 100 MJ/cm.sup.3 which are superior than other candidate materials such as boron nitride (BN), BNC, or silicon nitride (Si.sub.3 N.sub.4). Therefore, there is a need of a technique to manufacture the X-ray lithography mask using the SiC membrane.
The SiC membrane is usually formed by depositing SiC on a substrate by high temperature chemical vapor deposition (CVD) undertaken at a temperature in the range of 700.degree. C. to 1380.degree. C. The SiC membrane thus formed usually is made of a single crystal or polycrystal of SiC and is more stable against radiation than the amorphous SiC which is formed under a lower temperature condition by plasma CVD or ECR (electronic cyclotron resonance) processes although the latter has an excellent mirror surface associated with its amorphous structure.
The membrane of the crystalline SiC suitable for the X-ray exposure mask is typically deposited on a single crystal silicon (Si) substrate for a thickness of 2-3 .mu.m. Such a thick layer of SiC deposited on Si, however, has a problem in that formation of steps of about 0.1 .mu.m to 0.2 .mu.m on its surface is inevitable due to the mismatching of thermal expansion between Si and SiC or due to the grain boundaries appearing on the surface. Thus, the crystalline SiC membrane formed by the high temperature CVD process does not possess a completely defect-free mirror surface in the state that the membrane is formed.
In the X-ray exposure mask, a layer of material absorbing the X-ray such as tantalum (Ta) or gold (Au) is deposited on the SiC membrane and patterned as desired by selective etching. As the desired line width of the patterns in such an X-ray exposure mask is about 0.1 .mu.m to 0.2 .mu.m, the existence of surface irregularities having the magnitude about equal to this line width in the membrane is harmful as will be easily understood. Such irregularities are transferred on the semiconductor and decreases the yield of the manufactured semiconductor device.
Further, the applicants found that there exists a more serious problem in the conventional X-ray exposure mask of which reason is not understood at the moment. More specifically, when Ta is deposited on the SiC membrane as the material for absorbing the X-ray and thereafter patterned, it was found that Ta remains unetched particularly on the projected part of the irregular surface of the SiC membrane even after the patterning. On the contrary, Ta does not remain in the depressed part of the membrane surface, contrary to the commonly understood behavior. Thus, Ta remains on the surface of the SiC membrane where it should not exist. Such a membrane cannot be used for patterning the semiconductor devices.
Conventionally, there has been a number of efforts to grow the SiC membrane such that it has a mirror-flat top surface by controlling the condition of crystal growth. The term "mirror-flat" herein means a flat and smooth surface in which the step height of surface depressions or projections is less than at least 0.1 .mu.m or below the detection limit. However, such an approach has been so far not successful or effective.